Add support for emitting code with a single return at the end. (#153)

This also enables testing of the wasmtests tests.

This also updates for wabt updating to the official "wat" filename
extension, as opposed to "wast".

cranelift/wasmtests/icall.wat

@@ -0,0 +1,7 @@
+(module
+  (type $ft (func (param f32) (result i32)))
+  (func $foo (export "foo") (param i32 f32) (result i32)
+    (call_indirect $ft (get_local 1) (get_local 0))
+  )
+  (table (;0;) 23 23 anyfunc)
+)